Procedures have been developed for isolating nuclear RNA molecules, and RNA fragments derived from them, which contain the oligo U regions. This involves denaturation of the RNA with HCHO followed by poly A sepharose chromatography. This will permit the characterization of the nucleotide regions adjacent to the oligo U and possibly permit experiments which will indicate whether oligo containing HnRNA also contains mRNA sequences. The need for HCHO denaturation appears to be due to the inter and intramolecular interaction of the oligo U regions with the 3' poly A and internal oligo A sequences. This prompted a reinvestigation into the possible presence of oligo U in mRNA. HCHO denaturation of nonribosomal, polysomal RNA (mRNA) followed by poly A sepharose chromatography has permitted the separation of mRNA into four classes: 1) Poly A (plus) and oligo U (minus) (49 percent); 2) Poly A (plus) and oligo U (plus) (21 percent); 3) Poly A (minus) and oligo U (plus) (8 percent); 4) Poly A (minus) and oligo U (minus) (22 percent). The metabolic characteristics of these mRNAs is being explored. The ability to separate these mRNAs into four classes from which cDNAs can be synthesized, has initiated studies to determine the size of these mRNAs, their sequence relatedness to each other and which HnRNA molecules may be their precursors. BIBLIOGRAPHIC REFERENCES: Molloy, G.R. and Puckett, L. The metabolism of heterogeneous nuclear RNA and the formation of cytoplasmic messenger RNA in animal cells. Progress in Biophysics and Molecular Biology 31, 1 (1976). Sehgal, P.B., Derman, E., Molloy, G.R., Tamm, I. and Darnell, J.E. 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole inhibits initiation of nuclear heterogeneous RNA chains in HeLa cells. Science 194, 431 (1976).